This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Cytochrome c peroxidase (CCP) is an intercellular heme enzyme that oxidizes substrates in a multi-step reaction. The iron atom in Compound I is purportedly oxidized from +3 to +4 forming an oxyferryl center in the first step. Attempts to collect x-ray diffraction data from the intermediate compound I have been complicated due to auto-reduction of the complex from the incident x-rays We have been studying the auto-reduction rates using VIS spectroscopy. We have determined that at 67 K, ~4 exposures can be collected from moderately sized crystals to 1.3 A resolution. The temperature is just above the freezing point of nitrogen, while preserving 85% of Compound 1. Methods are being developed to use the robot to screen and collect data from a series of crystals to build a complete data set.